1. Field of the Invention
This invention relates to a process for the preparation of an ion conductive solid electrolyte. The ion conductive solid electolyte is favorably employable for the preparation of a solid electrolyte cell.
2. Description of Prior Art
A liquid electrolyte has been heretofore employed as an electrolyte for a primary cell, a secondary cell, an electrochromic display element, etc. However, the liquid electrolyte has a problem of confidence in storage for a long time, because a liquid or an electrode material sometimes leaks out of the cell or element.
A solid electrolyte does not have such a problem, and has an advantage that a constitution of a device is made simple. Further, it is possible to make parts of the device lightweight and small by forming the electrolyte into a thin film. Since these characteristic features of the solid electrolyte meet the requirements in electronics industry to supply small-sized, light-weight electronic parts and elements with high confidence. Accordingly, active studies have been made on improvement of a solid electrolyte.
It has been known that inorganic materials such as .beta.-alumina, silver oxide, rubidium, lithium iodide, etc. are employable as solid electrolytic materials. However, the inorganic material has problems in its practical use such that it is expensive and generally difficult to form the inorganic material into an optional shape such as a thin film.
A polymer has an advantage in that it can be formed into a uniform thin film of an optional shape. Therefore, it has been proposed to employ various polymers as material of solid electrolyte. For instance, solid electrolytes containing a composition comprising a combination of a polymer such as polyethylene oxide, polypropylene oxide, polyethyleneimine, polyepichlorohydrin, polyethylene succinate or the like and an inorganic ion salt such as a salt of Li, Na, or the like, as well as cells employing the composition are described in Japanese Patent Provisional Publication Nos. 55(1980)-98480, 58(1983)-75779, 58(1983)-108667, 58(1983)-188062, 58(1983)-188063 and 59(1984)-71263 and U.S. Pat. No. 4,576,882. However, these compositions are scarcely used in practice, because the ion conductivity of these compositions does not reach a sufficient level.
In the case using a solid electrolyte material, such low conductivity can be compensated by forming the electrolyte into a thin film. For example, where the solid electrolyte is used as a diaphragm of a cell, internal resistance can be reduced by forming the electrolyte into a very thin film.
As processes to form the solid polymer electrolyte into a thin film, there are described a process which comprises casting a solution of the solid polymer electrolyte and removing a solvent by evaporation or a process which comrises heat polymerization of a polymerizable liquid monomer coated on a base in Japanese Patent Provisional Publication Nos. 60(1985)-47372 and 60(1985)-49573.
However, the process for evaporation of the solvent is not advantageous from the viewpoint of working conditions, and moreover the heat polymerization needs a long time operation.
Japanese Patent Provisional Publication Nos. 59(1984)-149601 and 58(1983)-75779 describe that the insufficient ion conductivity of solid polymer electrolytes can be improved by incorporation of an organic solvent, keeping its solid condition. However, the organic solvent used in the polymer electrolyte is apt to vaporize or leak out of an electronic element using the electrolyte due to its low boiling point. Accordingly, the ion conductivity likely lowers in the course of the employment of the element or the element is likely damaged after its long term employment.
Solid State Ionics, vol., 11, 227 (1983) teaches a solid electrolyte composition comprising a polymethacrylic acid, lithium perchlorate and polyethylene glycol having a molecular weight of 400. Japanese Patent Provisional Publication 59(1984)-71263 proposes a solid electrolyte composition comprising polymethacrylate, a lithium salt and polyethylene glycol (or polypropylene glycol). Although these electrolytes are almost free from vaporization or leak-out of a component from the electrolyte composition, the level of ion conductivity is still unsatisfactory.
Japanese Patent Provisional Publication No. 58(1983)-82477 (corresponding to U.S. Pat. No. 4,556,614) proposes a solid electrolyte of a cross-linked network matrix comprising polyethylene oxide, an alkali salt and a cross-linkable polymer. However, it is required that the electrolyte is crosslinked to have the network matrix at an elevated temperature so as to keep its amorphous state, because the polyethylene oxide preferably employed in the art has such a high molecular weight of 5,000 to 7,000,000 so as to easily form in conjunction with the alkali salt a crystalline structure. Further, the ion conductivity of the obtained electrolyte is still unsatisfactory.